It is well known in the electronics art to place a heat sink in contact with an electronic device so that waste heat generated by operation of the electronic device is thermally transferred to the heat sink to cool the electronic device. However, with continued increases in areal densities and system clock speeds in electronic devices, such as microprocessors (CPU's), digital signal processors (DSP's), and application specific integrated circuits (ASIC), the amount of waste heat generated by those electronic devices and the operating temperature of those electronic devices are directly proportional to clock speed and device geometries. Efficient operation of a CPU as well as other high power dissipation electronic devices requires that waste heat be continuously and effectively removed.
However, as the aforementioned areal densities and system clock speeds of electronic devices continue to increase, a heat flux of the electronic devices also increases. Although air cooled heat sinks are commonly used to dissipate waste heat from the aforementioned electronic devices, the increased heat flux in high performance electronic devices is often concentrated in a small area, usually on a package surface that will be placed in thermal contact with the heat sink. The ability to effectively dissipate ever increasing levels of heat flux in high performance electronic devices has challenged current heat sink designs where the entire heat sink is fabricated using processes such as machining, forging, casting, and extrusion. Those processes make it difficult to increase the number of fins or an area of the fins in order to effectively dissipate heat flux concentration.
Heat flux is a thermal output per unit of area (i.e. W/cm2). For example, if a total thermal output is 100 Watts over a heat source having dimensions of 3.5 cm*3.5 cm, then the heat flux is 100 W÷(3.5 cm*3.5 cm)=8.163 W/cm2. At present, based on area and cost constraints, electronic device package size remains the same or decreases while the areal densities and clock speeds continue to increase. Consequently, the problems associated with heat flux concentration continue to increase and those problems cannot be solved solely by increasing heat sink size, the number of fins, fin surface area, or fan capacity.
Typically, waste heat from the heat mass is dissipated by an air flow through fins that are connected with the heat mass. However, with the increasing clock speeds and areal densities, a volume of an enclosure that houses the electronic device continues to decrease in many applications such as PC's, workstations, and servers, for example. Therefore, the aforementioned increases in heat sink size, the number of fins, fin surface area, or fan capacity are inconsistent with decreases in enclosure volume.
Turning to FIGS. 1a through 1d, a prior fan 500 for generating an air flow f to dissipate heat from a prior heat sink 550 includes a housing 501 and several apertures 504 for receiving a fastener (525, 526) (e.g. a nut and bolt) to mount the fan 500 on the heat sink 550. A component to be cooled (not shown) is placed in contact with a surface 519 of the prior heat sink 550. A prior mounting ring 521 can be used to mount the fan 500 over fins 551 of the heat sink 550. The fan 500 can include a stator 503 that is integrally formed with the housing 501 and a rotor 505 including several fan blades 507 is rotatably connected with the stator 503. The housing 501 has a thickness t, a width w, and a height h. The housing 501 serves as a shroud to protect the fan blades 507, as a platform for the stator 503, and as a frame to mount the fan 500 over the heat sink 550 such that a combined height HT of the combination (500, 521, 550) is t plus a height H1 (i.e. the combined height HT=H1+t). Therefore, increases in fan capacity often requires a larger size fan to increase capacity in CFM or an increase in fan speed in RPM. Accordingly, a larger fan size can increase the thickness t with a resulting increase in the combined height HT. In either case, the increased capacity comes at the expense of higher cost, increased fan noise, larger fan size, and increased energy consumption. Moreover, a larger size fan may not be possible if the combined height HT exceeds the design parameters for a given enclosure. Similarly, increasing a surface area of the fins 551, the number of fins 551, or a size of the heat sink 550 (e.g. the height H1 or a width W1) can result in increased cost, mass, and volume of the heat sink 550 and those increases may also exceed the design parameters for a given enclosure size.
Consequently, there is a need for a cooling system with increased fan capacity without increasing fan size or increasing fan noise. There is also a need for a cooling system with a fan with a reduced height so that the cooling system is smaller and can be accommodated in smaller enclosures. Finally, there exists a need for a cooling system with a fan that does not include a housing so that the fan has a small footprint, takes up less space, and can be flexibly positioned in a cooling device.